1. Field of the Invention
This invention relates to hydrotreating catalysts having a desirable combination of activity and high temperature stability and to processes for preparation and use thereof.
2. Description of the Prior Art
Increased concern over availability and security of petroleum crude oil supplies in recent years has focused considerable attention on producing and upgrading lower quality hydrocarbon feeds, such as synthetic crudes and heavy petroleum crude oil fractions. Unfortunately, high concentrations of nitrogen, sulfur, metals and/or high boiling components, for example, asphaltenes, resins, in such lower quality feeds render the same poorly suited for conversion to useful products in conventional petroleum refining operations, such as catalytic cracking and hydrocracking. For example, in catalytic cracking operations, nitrogen and metals tend to poison the cracking catalyst, sulfur leads to increased sulfur oxide emissions during catalyst regeneration, and high boiling components lead to formation of coke on the cracking catalyst which, in turn, can upset the heat balance of the catalytic cracking unit when the catalyst is regenerated. Hydrocracking catalysts also are poisoned by nitrogen and metals, and in addition, sulfur and coke promote deactivation thereof.
In view of such difficulties, lower quality hydrocarbon feeds often are catalytically hydrotreated to obtain materials having greater utility in conventional downstream refining operations. Catalytic hydrotreating involves contacting a feed with hydrogen at elevated temperature and pressure in the presence of catalysts having hydrogenation activity. As a result of such processing, sulfur and nitrogen in the feed are converted largely to hydrogen sulfide and ammonia which are easily removed. Aromatics saturation and, to a lesser extent, cracking of larger molecules often take place to convert high boiling feed components to lower boiling components. Metals content of the feed decreases as a result of deposition of metals on the surface of the hydrotreating catalyst.
Hydrotreating of low quality hydrocarbon feeds often is conducted under conditions more severe than those used in conventional hydrotreating of lighter hydrocarbon feeds in order to achieve suitable levels of nitrogen, sulfur and/or metals removal and/or conversion of high boiling components to lower boiling materials. For example, removal of nitrogen from high nitrogen feeds, such as whole shale oils or fractions thereof, typically requires higher temperatures and pressures and lower space velocities than those used in catalytic hydrotreating of low nitrogen feeds. Similarly, hydrotreating heavy petroleum crude oil fractions, such as vacuum or atmospheric resids and particularly those containing significant quantities of sulfur, metal and/or asphaltenes, usually requires operation under conditions more severe than those employed in hydrotreating lighter feeds.
As can be appreciated, satisfactory operation in processing feeds containing high levels of impurities under severe process conditions places increased demands on the catalyst to be employed as the same must exhibit not only high activity in the presence of impurities and under severe conditions, but also stability and high activity maintenance so that frequent replacement of catalyst is not required. Catalysts containing a Group VIB metal component, such as a molybdenum or tungsten component, promoted by a nickel or cobalt component and supported on a porous refractory inorganic oxide are well known and widely used in conventional hydrotreating processes; however, the same often are somewhat lacking in stability and activity maintenance under severe conditions.
Hensley et al., U.S. Pat. No. 4,297,242 have disclosed catalysts consisting of at least one active original hydrogenation metal selected from Group VIB deposed on a catalytically active support comprising alumina and use thereof with highly desirable results in hydrodesulfurization of heavy hydrocarbon feeds containing sulfur, nitrogen, metals and asphaltenes. Hensley et al. also disclose that such catalysts exhibit lower deactivation and improved lifetime, even under severe operating conditions, as compared to catalysts containing a Group VIB metal component promoted by a cobalt component.
Hensley et al., U.S. Pat. No. 4,212,729, which in its entirety is specifically incorporated herein by reference, disclose a two-stage catalytic process for hydrodemetallation and hydrodesulfurization of heavy hydrocarbon streams containing asphaltenes and a substantial amount of metals. The first stage of the process comprises contacting the feedstock in a first reaction zone with hydrogen and a demetallation catalyst comprising hydrogenation metal selected from Group VIB and/or Group VIII deposed on a relatively large-pore, high surface area inorganic oxide support. The second stage of the process comprises contacting the effluent from the first reaction zone with a catalyst consisting essentially of hydrogenation metal selected from Group VIB deposed on a relatively small pore, catalytically active support comprising alumina. The second stage catalyst has a surface area within the range of about 150 m.sup.2 /gm to about 300 m.sup.2 /gm, a pore volume within the range of about 0.4 cc/gm to about 0.9 cc/gm, an average pore diameter within the range of about 90 .ANG. to about 160 .ANG., and a majority of its pore volume in pore diameters within the range of about 80 .ANG. to about 130 .ANG.. More particularly, the pore volume distribution is such that less than 40% of its pore volume is in pores having diameters within the range of about 50 .ANG. to about 80 .ANG., about 45% to about 90% of its pore volume is in pores having diameters within the range of about 80 .ANG. to about 130 .ANG., and less than about 15% of its pore volume is in pores having diameters larger than 130 .ANG.. More preferably, the catalyst disclosed has a pore volume distribution summarized as follows:
______________________________________ Pore Diameter, .ANG. % of Pore Volume ______________________________________ 50-80 &lt;40 80-100 15-65 100-130 10-50 &gt;130 &lt;15 ______________________________________
In terms of the surface area, the pores of the disclosed catalyst having diameters within the range of about 80 .ANG. to about 130 .ANG. preferably contain about 90 to about 180 m.sup.2 /gm, and more preferably contain about 120 to about 180 m.sup.2 /gm, of surface area.
It has also been found that addition of a chromium component to catalysts comprising a Group VIB metal component alone or promoted by a Group VIII metal component gives highly desirable results in a wide range of applications including high severity hydrotreating applications. Such catalysts and processes for use thereof are disclosed in Quick et al., U.S. Pat. No. 4,181,602; Quick et al., U.S. Pat. No. 4,188,284; Quick et al., U.S. Pat. No. 4,191,635; Hensley et al., U.S. Pat. No. 4,224,144; Hensley et al., U.S. Pat. No. 4,278,566; and Hensley et al., U.S. Pat. No. 4,306,965.
It has long been known that preparation of hydrotreating catalysts containing Group VIB and Group VIII metal components supported on a porous refractory inorganic oxide can be improved through the use of phosphoric acid impregnating solutions of precursors to the Group VIB and Group VIII metal components or the use of phosphoric acid as an impregnation aid for the metal precursors. Thus, Pessimisis, U.S. Pat. No. 3,232,887 discloses stabilization of Group VI and Group VIII metal-containing solutions through the use of water-soluble acids. According to the patentee, in column 3, lines 6-11, "in its broadest aspect the invention comprises the preparation of stabilized aqueous solutions which comprise an aqueous solvent having dissolved therein catalytically active compounds containing at least one element from Group VI of the periodic table and one element from Group VIII." Inorganic oxyacids of phosphorus are included among the disclosed stabilizers, and the examples of Pessimisis illustrate preparation of various cobalt-molybdenum, nickel-molybdenum, and nickel-tungsten catalysts using phosphorus and other acids as stabilizers. Hydrodesulfurization results with certain of the cobalt-molybdenum catalysts are presented, and the patentee suggests that the use of the stabilized solutions may lead to improved hydrodesulfurization activity in some instances.
Similarly, Colgan et al., U.S. Pat. No. 3,287,280 discloses the use of phosphoric acid as an impregnation aid in preparation of nickel-molybdenum catalysts and that such use can result in catalysts having improved hydrodesulfurization activity.
Kerns et al., U.S. Pat. No. 3,446,730 disclose hydrodenitrogenation catalysts comprising at least one of a nickel component and a Group VI metal component, supported on a specific alumina, such catalysts being promoted with 0.1 to 2.0 wt. % of a promoter selected from compounds of phosphorus, silicon and barium. However, at column 3, lines 26-38, the patentees make the following remarks with respect to promotion of hydrogenating metals on the specific alumina disclosed, as follows:
"The nature of the hydrogenation component of the composite catalysts disclosed herein is very important, as we have found that not every hydrogenation component, when composited with the herein disclosed aluminas, is susceptible to promotion as disclosed herein. For example, cobalt-containing catalysts prepared from the special aluminas disclosed herein are unsuitable for purposes of this invention, not-withstanding that cobalt is often a component of conventional denitrogenation catalysts. In fact, our experiments indicate that the denitrogenative activity of catalysts comprising cobalt and the special activated aluminas disclosed herein are actually lessened by the addition thereto of phosphorus, silicon or barium." PA1 "In addition, however, phosphoric acid must not be present in the impregnating solution in an amount which upon subsequent calcination of the catalyst material will adversely affect the activity and strength of the catalyst in use and upon repeated regenerations to any substantial extent."
Hilfman, U.S. Pat. No. 3,617,528 discloses hydrotreating catalysts comprising coextruded nickel and phosphorus components and an alumina-containing support. The catalysts also may contain a Group VIB metal component.
Mickelson et al., U.S. Pat. No. 3,706,693 and Hass et al., U.S. Pat. No. 3,725,243 disclose hydrotreating catalysts prepared by forming an intimate admixture of an amorphous, foraminous, refractory oxide containing a substantial proportion of alumina with at least one crystalline, ion-exchangeable aluminosilicate containing less than about 5 wt. % alkali metal oxides, and contacting the result with an aqueous acidic solution of at least one Group VIII metal compound, at least one Group VI metal compound and at least one phosphorus acid at a pH below about 3 under conditions effective to deposit catalytic amounts of the metals on the refractory oxide and react at least a portion of the aluminosilicate with the aqueous acidic medium. According to the patentees, the disclosed catalysts are more tolerant of nitrogen than catalysts prepared without an aluminosilicate component. Further, Examples 10-13 of both patents illustrate catalysts exhibiting improved hydrodenitrogenation activity as compared to catalysts prepared without an aluminosilicate component. Reported hydrodesulfurization activity is slightly worse.
Mickelson, U.S. Pat. Nos. 3,749,663; 3,749,664; 3,755,150 and 3,755,196 disclose catalysts comprising molybdenum, at least one Group VIII metal component and phosphorus deposed on a refractory inorganic oxide support. The '664 patent is directed specifically to the use of such catalysts for hydrodenitrogenation.
Colgan et al., U.S. Pat. No. 3,840,472 disclose catalysts prepared by impregnation of an alumina support with stabilized solutions of molybdic oxide and certain cobalt or nickel salts dissolved in aqueous phosphoric acid although the patentees suggest that the presence of certain amounts of a phosphorus component in the ultimate catalyst may harm performance, stating the following at column 2, lines 23-28:
Simpson, U.S. Pat. No. 4,255,282 discloses hydrotreating catalysts comprising molybdenum, nickel, and phosphorus components and a gamma-alumina support, such catalysts being prepared by a method that involves a precalcination of the gamma-alumina at a temperature greater than 746.degree. C. With respect to the phosphorus component, Simpson teaches that the same often has been included in hydrotreating catalysts to increase catalyst acidity and thereby improve activity.
Ripperger and Saum, Chemistry and Uses of Molybdenum, Proceedings of the Second International Conference, pages 175-179 (1976), report that addition of phosphoric acid to a catalyst consisting of nickel and molybdenum supported on alumina resulted in increased hydrodenitrogenation activity but that effective promoters could not be found for catalysts consisting of cobalt and molybdenum on alumina.
While the patents and publication discussed above disclose that the use of phosphoric acid in the preparation of hydrotreating catalysts containing Group VIB and Group VIII metal components is beneficial to the preparations, reported effects on catalytic activity and performance vary significantly. For example, the general statement in the aforesaid Simpson patent regarding use of a phosphorus component to increase acidity and thereby improve activity is contrary to the teaching of Colgan, U.S. Pat. No. 3,840,472 that use of phosphoric acid in improper amounts can adversely affect catalyst activity and strength. More specifically, while the aforesaid Pessimisis patent and Colgan et al., U.S. Pat. No. 3,287,280 attribute to use of phosphoric acid in catalyst preparation, or to phosphoric acid residue content in finished catalysts, promotional effects in respect of hydrodesulfurization activity of cobalt-molybdenum and nickel-molybdenum catalysts, and while certain of the aforesaid Mickelson patents illustrate a similar influence on hydrodenitrogenation activity of Group VI and Group VIII metals-containing catalysts, the aforesaid Ripperger and Saum article teach that phosphoric acid use leads to improved hydrodenitrogenation activity for nickel-molybdenum catalysts but not for cobalt-molybdenum catalysts. Further, the aforesaid Kerns et al. patent teaches that hydrodenitrogenation activity of cobalt-containing catalysts in general, and nickel-molybdenum-cobalt catalysts in particular, decreases when the specific alumina support disclosed therein is employed and when a phosphorus promoter is used.
Notwithstanding the diverse teachings of the aforesaid patents and publication in respect of stabilization and promotion of hydrotreating catalysts, there is a continuing need for development of improved catalysts.